This invention relates to the protection of substrates, such as against wear damage and, more particularly, to the use of layered quasicrystalline-ductile metal coatings to provide that protection.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
A number of structures in the gas turbine engine are subjected to conditions of wear at temperatures ranging from ambient to moderately elevated. In wear, the contacting surfaces of two components rub against each other in point, line, or full-surface contact. Typical results include scoring of one or both surface, and possibly metal removal from one or both surfaces. As the surfaces are damaged, they become even more susceptible to the effects of wear as their effective coefficients of friction rise and wear debris is trapped between the wearing surfaces, so that the wear damage accelerates with increasing time in service.
The wear condition sometimes arises because it is not desirable or possible to firmly affix the two components together to prevent the rubbing action, because of the functionality of the components. An example is a cylindrical bushing used to support a variable stator vane in the compressor section of the gas turbine engine, wherein the element inserted into the bushing rotates or slides in contact with the surface of the bushing. Another example is the dovetail of a compressor, bypass-fan, or gas turbine blade, which must fit loosely into its dovetail slot of the rotor upon which it is mounted due to considerations of loading and expansion, and therefore rubs against the dovetail slots during service.
In most cases, the materials of construction of the rubbing components must be selected for reasons other than wear resistance. The rubbing components therefore are often susceptible to wear damage. In some cases, reduced wear damage may be realized by applying hard coatings such as tungsten carbide-cobalt, carbide, or nitride material to one or both of the contacting surfaces, or by hardening one or both of the contacting surfaces with a surface treatment such as carburizing or nitriding. However, these approaches have shortcomings, primarily associated with an insufficient service life at elevated service temperatures. The coating or surface treatments may be affected by pilling, galling, or the like. In some cases the coating or surface treatments are subject to environmental damage such as oxidation and/or corrosion. Additionally, the use of hard coatings and surface-hardening treatments may lead to reduced fatigue life of the components by serving as the source of surface cracks that are formed in the hard surface layers and propagate into the components as they are cyclically loaded during service.
There is accordingly a need for an improved approach to the protection of gas turbine components, such as bushings and dovetail surfaces, and other articles as well, against the damage caused by wear. The present invention fulfills this need, and further provides related advantages.
The present invention provides an approach for preparing an article having a layered coating thereon. The layered coating is particularly effective in protecting a substrate against the effects of wear damage and may be optimized for this use as described herein, although it is not limited to this use. The coating of the present approach is further tailored to minimize the possibility that cracks in the coating can propagate into the underlying substrate to cause it to fail prematurely, as by a fatigue mechanism.
A coated article comprises a substrate; and a layered coating overlying the substrate. The coating comprises a ductile metallic layer in facing contact with the substrate, and a protective layer overlying and in facing contact with the ductile metallic layer. The ductile metallic layer and the protective layer are both preferably substantially continuous. The protective layer in turn comprises a mixture of a quasicrystalline metallic phase and a non-quasicrystalline ductile phase. Preferably but not necessarily, the quasicrystalline metallic phase is present in the protective layer in an amount of from about 90 volume percent to about 99 volume percent, most preferably embedded in the non-quasicrystalline ductile phase which serves as a matrix. Preferably but not necessarily, the protective layer has a thickness of from about 10 to about 100 micrometers, and the ductile metallic layer has a thickness of from about 5 to about 10 micrometers.
In an application of interest, the substrate is a component of a gas turbine engine. The substrate may be, for example, a bushing or a dovetail of a blade.
The quasicrystalline metallic phase, which is a relatively hard, brittle material, may be any operable material. Following the usual convention in the art, the term xe2x80x9cquasicrystallinexe2x80x9d as used herein includes both pure-quasicrystalline compositions and also approximant-quasicrystalline compositions. Alloys of most current interest include an alloy comprising iron, copper, and aluminum, an alloy comprising iron, cobalt, chromium, and aluminum; an alloy comprising nickel, cobalt, chromium, and aluminum; an alloy comprising titanium, zirconium, nickel, and silicon; and an alloy comprising titanium, nickel, and zirconium. The ductile metallic layer may be any operable material that is not a quasicrystalline metal, but desirably is a metal such as an iron-base alloy, a nickel-base alloy or a titanium-base alloy. The ductile metallic layer is preferably, but not necessarily, a different metal than the substrate. It is preferred that the protective layer and the ductile metallic layer each are of about the same coefficient of thermal expansion, and about the same coefficient of thermal expansion as the underlying substrate, to minimize differential thermal expansion thermal stresses and strains resulting from temperature changes during fabrication and during service.
A method for providing a coated article having a high resistance to wear damage comprises the steps of providing a substrate, and applying a layered coating overlying the substrate to form the coated article. The coating comprises a ductile metallic layer in facing contact with the substrate, and a protective layer overlying and in facing contact with the ductile metallic layer. The protective layer comprises a mixture of a quasicrystalline metallic phase and a non-quasicrystalline ductile phase. The coated article is subjected to wear conditions. Operable features and modifications of the approach discussed elsewhere may be utilized in this embodiment as well.
The layered coating includes the relatively hard composite protective layer to provide good wear resistance. This composite protective layer includes the quasicrystalline metallic phase and the non-quasicrystalline ductile phase, which is typically a metal (which for the present purposes includes conventional metals and metal-like intermetallic compounds such as FeAl). The non-quasicrystalline ductile phase improves the fracture toughness of the protective layer, as compared with a layer having only the quasicrystalline metallic phase, and also improves its friction and wear properties.
The ductile metallic layer serves as a bond coat to ensure the adhesion of the protective layer to the substrate. The ductile metallic layer also has the beneficial effect of preventing cracks that may initiate in the less-ductile protective layer and specifically in the quasicrystalline metallic phase from propagating inwardly to the substrate, and thence causing premature cracking of the substrate. Any such cracks are blunted and deflected when they reach the ductile layer.
The use of the present layered coating provides a significant improvement in resistance to wear damage as compared with an unprotected substrate article. The present layered coating also has important advantages as compared with conventional protective coatings such as the commonly used tungsten carbide-cobalt nonlayered coating. The present layered coating has significantly lower density than the tungsten carbide-cobalt coating, and a better match to the coefficient of thermal expansion of the substrate in most cases. The quasicrystalline exposed layer of the layered coating has good environmental performance, such as elevated temperature corrosion resistance and oxidation resistance.
Another important advantage of the present approach is that the layered coating is operable to the moderately elevated temperatures to which many components of an aircraft gas turbine engine are subjected during service. The maximum temperature depends upon the composition of the wear coating, but is as high as 1000xc2x0 C. for some quasicrystalline materials.